1. Field of the Invention
This invention relates to a method for manufacturing a suspension spring used in an automotive vehicle and more particularly to a method for manufacturing a coiled spring having a high fatigue strength.
2. Description of the Prior Art
In coiled springs used as suspension springs in cars, for car body weight reduction, high fatigue strength is being required. In a known method for manufacturing a coiled spring having a high fatigue strength, a high tensile strength rod material made by drawing, quenching and tempering a spring steel rod is used; the rod material is subjected to the steps of coiling, heat-treating, grinding, and shot peening to impart residual stress thereto and then to the step of polishing to reduce surface roughness thereof.
A method for manufacturing a coiled spring is disclosed, for example, in Japanese Patent Laid-Open Publication No. HEI-3-037434. The manufacturing steps of this coiled spring manufacturing method are shown in FIG. 4 hereof and are as follows:
In a cold coiling step ST100, a coil rod is coiled to a predetermined diameter.
In a first low-temperature annealing step ST101, the coil is annealed at a low temperature.
In an end-grinding step ST102, the ends of the coil are ground.
In a shot peening step ST103, the coil is given a compressive residual stress.
In an identification paint coating step ST104, the coil is coated with paint.
In a second low-temperature annealing step ST105, the coil is annealed at a low temperature again.
And in a hot setting step ST106, a predetermined load is applied to the coil for 15 seconds at a temperature of 250xc2x0 C.
In this coiled spring manufacturing method, to raise the residual compressive stress in the coil by shot peening, it is necessary to increase the particle diameter of the shot, to raise the surface hardness of the shot or to increase the shot speed; however, when this is done, the issue arises that because the surface roughness of the coil increases and consequently stress actually tends to concentrate rather than disperse, the fatigue strength falls.
Another coiled spring manufacturing method is disclosed in Japanese Patent Laid-Open Publication No. HEI-8-41533. This coiled spring manufacturing method is shown in FIG. 5 hereof and is made up of the following steps:
In a step ST110, an oil-tempered steel rod is annealed at a high temperature to obtain an annealed rod.
In a cold coiling step ST111, the annealed rod is coiled by cold working.
In a step ST112, the coiled spring is quenched and tempered.
In a step 113, a seat face (for bearing a compressive load when the coiled spring is compressed) of the coiled spring is ground.
In a step ST114, gas nitriding treatment is carried out on the coiled spring in an ammonia gas atmosphere.
In a step ST115, two-stage shot peening is carried out on the coiled spring:
In a step ST116, a first shot peening of the two-stage shot peening step is carried out. That is, impeller-blasting of cut wire of particle diameter 0.6 to 1.0 mm and Vickers surface hardness 650 to 850 is carried out at a shot speed of 70 to 100 m/s.
In a step ST117, a second shot peening of the two-stage shot peening step is carried out. That is, air-blasting of cut wire or steel balls of particle diameter 0.15 to 0.3 mm and Vickers surface hardness 700 to 900 is carried out at an air pressure of 0.3 to 0.7 MPa.
And in a step ST118, low-temperature annealing is carried out to remove internal strain in the coiled spring.
However, with this coiled spring manufacturing method, although a two-stage shot peening is carried out, by this step alone it is difficult to provide a compressive residual stress from the surface of the coil to deep positions, and it is not possible to raise the fatigue strength sufficiently.
It is therefore an object of the present invention to provide a manufacturing method for a car suspension spring by which it is possible to manufacture a coiled spring having a high fatigue strength of endurance limit (xcfx84m) 687xc2x1588 MPa.
To achieve this object and other objects, the invention provides a method for manufacturing a suspension spring for use in an automotive vehicle, comprising: a cold coiling step of cold forming a rod of tensile strength 1910 to 2020 N/mm2, rod diameter 8 to 17 mm into a coil; a strain-removing step of annealing the coil to remove strains arising in the coil during forming of the coil in the cold coiling step; a hot setting step of compressing the coil by applying a predetermined load thereto at a temperature higher than room temperature and holding that state; and a shot peening step of carrying out a multi-stage shot peening on the coil.
As a result of a hot setting step and a multi-stage shot peening step being combined in this way, in the hot setting step a compressive residual stress is provided in a position deep from the surface of the coil and in the multi-stage shot peening step a higher compressive residual stress is provided in a position near to the coil surface, and a suspension spring having a high fatigue strength is obtained.
In the hot setting step, the coil is compressed with a load at least 10% greater than the maximum in-use load of the coil. For example if a load less than 10% greater than the maximum in-use load of the coil is used, it is not possible to provide satisfactorily a compressive residual stress in a position deep from the coil surface. This hot setting step is carried out utilizing surplus heat from the preceding strain-removing step. When surplus heat is utilized in this way it is not necessary for heating means to be provided separately in the hot setting step, and manufacturing cost can be reduced.
The multi-stage shot peening step preferably includes at least a first stage shot peening wherein steel balls or cut wire of surface Vickers hardness 550 to 650 and particle diameter 0.6 to 1.0 mm are used as a first shot material and the shot speed of the first shot material is 60 to 90 m/s and a second stage shot peening wherein steel balls or cut wire of surface Vickers hardness 600 to 800 and particle diameter 0.15 to 0.3 mm are used as a second shot material and the shot speed of the second shot material is 60 to 90 m/s. That is, the fatigue strength of the coil is raised by a compressive residual stress being provided in the first stage shot peening at relatively deep positions in the vicinity of the coil surface and a compressive residual stress being provided in the second stage shot peening at positions nearer to the coil surface than in the first stage shot peening. Also, by the particle diameter of the shot material used in the second stage shot peening being made smaller than the particle diameter of the shot material used in the first stage shot peening, roughening of the surface of the coil is suppressed and consequent decreasing of the fatigue strength of the coil is prevented.
Preferably, in the first stage of the multi-stage shot peening step the minimum dose of shot material blasted per unit area from the start of the shot peening to the finish is made 180 kg/m2 and in the second stage the minimum shot dose is made 100 kg/m2. For example if the dose of shot material blasted in the first stage shot peening is less than 180 kg/m2, the compressive residual stress arising at positions relatively deep from the surface of the suspension spring becomes small. And if the dose of shot material blasted in the second stage shot peening is less than 100 kg/m2, the residual stress arising at positions near to the surface of the suspension spring becomes small. Either case is undesirable, because the fatigue strength of the suspension spring decreases and it becomes impossible to satisfy predetermined endurance conditions.